Optical fiber has become a widely accepted form of transmission media. The use of optical communications involving the use of optical fibers has grown at an unprecedented pace. A continuous length of the fiber is drawn from an optical preform which may be made by any one of several known processes. Afterwards, or as part of a tandem process, the drawn fiber is coated, cured, measured and taken up, desirably in an automatic takeup apparatus, on a spool. Typically, an optical fiber has a diameter on the order of 125 microns, and is covered with coating material which increases the outer diameter of the coated fiber to about 250 microns, for example.
The spool on which the optical fiber is taken up has other uses. It is used to store the optical fiber, to pay out and to take up the fiber for other operations such as ribboning, cabling, and rewinding and is used to ship optical fiber which is wound thereon to other companies which further process the fiber. Also, it may be used in weapons and communications systems in which it may be attached to a control station.
Weapons and communications systems which use an optical fiber for two-way data communication between two or more moving bodies or between a moving body and a guidance station have been developed or are under development. Such uses include communication lines between aircraft, between an aircraft and a ship, and between a projectile, such as a missile, and a control station at a launch site, for example. Advantageously, the use of optical fiber for these kinds of communication precludes electromagnetic interference and undesired interception.
There are, however, in using optical fiber certain disadvantages, not present in other forms of communication. Optical fiber is less robust than metallic conductors, rendering it subject to breakage. Aside from breakage, optical fiber communication performance may be degraded by microbends in the fiber which are generated by bending or by other stresses to which the fiber is subjected. Such damage to an optical fiber not only reduces the long-term durability of the fiber, but also causes losses in the strength and in the content of the optical signal.
A typical optical fiber application in a weapons systems involves the packaging of a continuous length of optical fiber on a carrier bobbin which is positioned inside a vehicle. Such a vehicle commonly is referred to as a tethered vehicle. One end of the fiber is attached to operational devices in the vehicle, whereas the other end of the fiber is connected to a control or communications station at a launch site. During and after launch, two-way communication with the vehicle is conducted.
In order to use such an arrangement, there must be provided a reliable and compact package of the optical fiber which may be disposed within the vehicle and which will permit reliable deployment of the optical fiber during the flight of the vehicle. The use of metallic conductors for guidance or control of launched vehicles is known. See, for example, U.S. Pat. Nos. 3,114,456, 3,156,185 and 3,319,781. As mentioned hereinabove, the characteristics of optical fiber present difficulties not involved in the use of metallic conductors for communication. Specialized treatment is required to facilitate the unwinding of the optical fiber from its carrier bobbin at a relatively high rate of speed.
A problem in the optical fiber guidance of tethered vehicles relates to the successful unwinding of the fiber from a carrier bobbin as the bobbin is propelled along with the vehicle. The leading end of the optical fiber is connected to a guidance system for controlling the path of travel of the vehicle. It becomes important for the optical fiber to be payed off from the bobbin without the occurrence of snags, otherwise the fiber may break and the control system rendered inoperable. Contributing to the successful payout of the optical fiber is a precision wound package. Further, not only must the convolutions be wound with precision, they also must remain in place as wound during handling and when deployed. In other words, the optical fiber package must be a highly stable one. On the other hand, payout must occur easily without the necessity of high pulling forces to remove each convolution of fiber from the carrier bobbin.
In some optical fiber packages for use in tethered vehicles, many layers of optical fiber are wound on a base layer of wire. An adhesive material between the optical fiber turns functions to hold the package together, forming a stable structure which is resistant to environmental extremes, shock and vibration. Desirably, the adhesive material which is used to hold together the convolutions must have a minimal impact on the optical performance of the wound optical fiber, and yet it must allow the optical fiber to be payed out with a controlled force at the peel-off point as the outermost turn is unwound at high speed. These requirements present somewhat conflicting requirements for the adhesive system.
During storage and transport of the carrier bobbin, mechanical stability is most important as the adhesive adds integrity to the wound package thereby maintaining the package in a ready condition for deployment. During deployment, both mechanical and optical effects are significant. The adhesive system must provide tackiness which is sufficiently low to permit a helical pattern of payout at speeds which may be relatively low to speeds which may be in the supersonic range. Excessive tackiness threatens fiber integrity by forming an extreme bend at the peel-off point. On the other hand, not enough tack may result in failure through dynamic instability on the bobbin surface. With respect to optical performance, optical attenuation at the peel off-point of each successive convolution may occur through localized macrobending, degrading the integrity of data and video transmission. Typical peel-off point attenuation of each successive convolution may contribute 3 or more dB to the overall loss.
Also, it has been found that microbending in the layers of undeployed fiber in the bobbin during deployment can affect adversely optical performance. It has been found that the adhesive material can contribute significantly to attenuation increases, especially at lower temperatures.
Current techniques for providing a sought-after stable package include providing a length of optical fiber to be wound with an adhesive material which is not tacky at room temperature but which becomes tacky at a predetermined temperature. After the optical fiber has been precision mound wound on a bobbin, the bobbin is subjected to the predetermined temperature to cause the adhesive material to become tacky and cause each convolution to adhere to at least a portion of adjacent convolutions. The adhesion is sufficient to cause a precision wound package to be maintained, but is such as to allow separation of convolutions during payout without the occurrence of breaks. See U.S. Pat. No. 4,950,049 which issued on Aug. 21, 1990 in the names of R. J. Darsey, J. W. Shea, and C. R. Taylor.
Although the just-described arrangement overcomes the problem of providing a stable package which gives acceptable payout, the process of providing the package requires the steps of applying an adhesive material to portions of the outer surfaces of the convolutions and curing same. What is needed and what seemingly is not available in the prior art is a package of optical fiber suitable for high speed payout which is provided with reduced processing steps. What is desired is a package of optical fiber suitable for high speed payout and capable of being manufactured with minimum incremental cost over conventionally made optical fiber.